A color Doppler method in ultrasonic diagnosis extracts blood flow component information by shifted frequencies due to the Doppler effect upon irradiating a living body with ultrasonic waves a plurality of number of times in the same direction. Blood flow component information includes a blood flow velocity, power, and variance. For example, as shown in FIG. 11, this method applies ultrasonic waves in the same direction (to the same raster) N times at a period of 1/PRF (PRF: Pulse Repetition Frequency). The data string in which N echo components corresponding to the same direction and same depth are arrayed along the time axis is called an ensemble direction or Doppler direction. The depth direction is also called the distance direction. Ensemble data is Fourier-transformed, thereby obtaining Doppler frequencies.
The intensity of a tissue signal (clutter component signal) is higher than the signal intensity of blood flow components by about 40 to 100 dB. In order to extract blood flow components flowing in the tissue, it is necessary to suppress tissue signals. A high-pass filter called a wall filter suppresses clutter components. An apparatus is required to have an S/N of 100 dB or more.
A living body has frequency-dependent attenuation. Even when ultrasonic waves are transmitted in a wide band, the center of an echo signal has a low-frequency band. This makes it difficult to use the energy of ultrasonic waves with a high S/N ratio at the time of reception even when they are transmitted in a wide band. For this reason, in color Doppler imaging which requires a high S/N ratio, ultrasonic waves are transmitted in a narrow band.
Recently developed apparatuses have achieved high S/N ratios. Even in color Doppler imaging, such apparatuses can finely express blood flow components in a wide band as in B-mode imaging even when ultrasonic waves are transmitted in a wide band. A wider band is effective in improving distance resolution.
In the color Doppler mode, however, the following problem arises when ultrasonic waves are transmitted in a wide band.
Letting fRF be the frequency of an ultrasonic wave and c be the velocity of sound, when a reflector moves in a direction to approach the probe at a constant velocity v, a frequency fDop of the resultant Doppler shift is given by
                              f          Dop                =                                            2              ⁢              v                        c                    ⁢                      f            RF                                              (        1        )            
That is, since the Doppler frequency is proportional to the transmission frequency of an ultrasonic wave, for example, the Doppler frequency obtained by a 2-MHz ultrasonic wave differs twice from the Doppler frequency obtained by a 4-MHz ultrasonic wave. The graph on FIG. 12 shows the relationship between the ultrasonic frequency and the Doppler frequency. The abscissa represents the Doppler frequency; and the ordinate, the ultrasonic transmission frequency. Assume that the amplitude of a Doppler signal at a given Doppler frequency and ultrasonic frequency is expressed by the third axis (not shown) perpendicular to the above two axes. Both the Doppler frequency of a blood flow component and the Doppler frequency of a clutter component are proportional to an ultrasonic frequency.
The prior art performs signal processing for only a Doppler frequency. For this reason, the frequency axes on the above portion of FIG. 12 are integrated into one, and signal processing is then performed for a signal in which blood flow components partially overlap clutter components. The abscissa of the lower graph of FIG. 12 represents the Doppler frequency; and the ordinate, the amplitude of a Doppler signal. That is, although the blood flow components are separated from the clutter components, their Doppler frequencies overlap on the frequency axis, which cannot be separated from each other.
In narrow-band transmission/reception, which has been conventionally used, since only frequencies near a center frequency f0 of a transmission ultrasonic frequency exist, the Doppler frequencies of blood flow components and clutter components do not overlap. However, when ultrasonic waves are transmitted/received in a wide band, the Doppler frequency components of blood flow components and clutter components may overlap. In such a case, a wall filter cannot separate clutter components from blood flow components, resulting in an image containing many clutter components.
In addition, when ultrasonic waves are transmitted in a wide band with the center frequency f0, an actual reception signal is influenced by frequency-dependent attenuation in the living body and is received at a frequency f1 lower than f0. In general, however, f1 cannot be known. Equation (1) is therefore rewritten into
                    v        =                              2            c                    ⁢                                    f              RF                                      f              Dop                                                          (        2        )            In equation (2), since fRF is unknown, it is not possible to obtain a correct velocity v. In transmission/reception of ultrasonic waves in a narrow band, which has conventionally been used, since the transmission frequency f0 is almost equal to the reception frequency f1, such a problem has not arisen.
Patent reference 1 discloses a method using two types of ultrasonic transmission/reception frequencies. This patent reference discloses the method of estimating a velocity exceeding an aliasing velocity by using different ultrasonic transmission/reception frequencies and the fact that blood flow components with the same velocity differ in Doppler frequency. However, the reference has no mention of a technique of separating clutter components from blood flow components.